Toxic gas indicator

ABSTRACT

A toxic gas detection and alarm system comprising a variable resistance semiconductor type sensor connected in a bridge circuit and compared to a reference voltage to control a relay circuit for an alarm device. The voltage comparison is carried out by means of an amplifier and switch arrangement having a positive feedback circuit for controlling a &#39;&#39;&#39;&#39;hysteresis&#39;&#39;&#39;&#39; effect in the overall detector circuit sensitivity. An autotransformer having a tapped primary and a pair of secondary windings is used to energize the sensor and to provide the bridge reference voltage such that turning the alarm device on results in increased sensor energization current. This increase in energization causes a decrease in sensor sensitivity and an automatic purging of the sensor area.

United States Patent [191 Swigert June 3, 1975 TOXIC GAS INDICATOR [75]Inventor: David L. Swigert, Ypsilanti, Mich.

[73] Assignee: Environmental Metrology C0rp.,

Ann Arbor, Mich.

[22] Filed: May 7, 1973 [21] Appl. No.: 357,631

[52] US. Cl. 340/237 R; 340/285 [51] Int. Cl. G08b 17/10 [58] Field ofSearch 340/237 R, 237 S, 381,

340/285; 23/232 E, 254 E, 255 E; 73/27 R [56] References Cited UNITEDSTATES PATENTS 3,251,654 5/1966 Palmer 340/237 R Primary ExaminerJohn W.Caldwell Assistant ExaminerDaniel Myer Attorney, Agent, or Firm-ThomasN. Young 57 ABSTRACT A toxic gas detection and alarm system comprising avariable resistance semiconductor type sensor connected in a bridgecircuit and compared to a reference voltage to control a relay circuitfor an alarm device. The voltage comparison is carried out by means ofan amplifier and switch arrangement having a positive feedback circuitfor controlling a hysteresis effect in the overall detector circuitsensitivity. An autotransformer having a tapped primary and a pair ofsecondary windings is used to energize the sensor and to provide thebridge reference voltage such that turning the alarm device on resultsin increased sensor energi- Zation current. This increase inenergization causes a decrease in sensor sensitivity and an automaticpurging of the sensor area.

7 Claims, 3 Drawing Figures TOXIC GAS INDICATOR INTRODUCTION Thisinvention relates to toxic gas alarms and particu larly to an improveddetector and alarm system using a solid state sensor device whichpresents a varying electrical resistance characteristic in response tothe presence of a toxic gas such as carbon monoxide, propane, butane,alcohol, gasoline and other hydrocarbons.

BACKGROUND OF THE INVENTION The growing awareness of the harmful effectsof a high toxic gas level in the air has resulted in an increased needfor sensitive and reliable devices for detecting the presence of suchgases in dangerous quantities and for providing a suitable indicationwhen an alarm condition exists. Some early prior art devices haveemployed heated platinum wires for burning the gases and suitabledetector means for indicating the burning condition. However, such priorart devices consume substantial amounts of power and thus do not lendthemselves to situations and applications where a low power portable orsemiportable device is required.

Later, improved prior art devices include substantially improvedsemiconductor type sensors which present a varying electrical resistancecharacteristic in response to the presence of the toxic gas, but suchsensors have heretofore been primarily combined with associatedcircuitry which produces a relatively low sensitivity as well as otherproblems such as line voltage sensitivity and instability. One suchprior art circuit includes an SCR switch connected to receive thevarying voltage which results from the variable resistancecharacteristic to switch current through a relay which in turn controlsan alarm device. It can be readily appreciated that such a detectorcircuit is sensitive to line voltage variations and in fact may createan instability problem in that the switching of the alarm device to theenergized condition inherently results in a line voltage drop across thesensor. In addition, it has been found that the typical metal oxide typesolid state sensor exhibits an alternating current component in theoutput signal which can aggravate the instability problem unlesssuitable compensation means are employed.

. Accordingly there exists a need for a substantially improved toxic gassensor using semiconductor type sensors so as to provide increasedreliability, simplicity and sensitivity to a much greater degree thanhas heretofore been available.

BRIEF SUMMARY OF THE INVENTION In accordance with the present inventiona toxic gas detector and alarm system is provided wherein thesensitivity to line voltage variations of the prior art devices iseffectively eliminated. In general this is accomplished by means of adetector circuit for use in combination with a semiconductor, variableresistance type sensor, the sensor being connected as a component in abridge circuit such that an electrical signal quantity related to thetoxic gas presence is compared to a reference signal. In the preferredembodiment, the electrical signal quantity and the reference signal aregenerated from the same supply line thereby to maintain a constantamplitude ratio irrespective of line voltage variations.

In accordance with a second feature of the present invention, means areprovided to effectively eliminate any cycling effect which mightotherwise result from the AC component of the sensor signal and forproviding, in addition, a variable hysteresis effect to accommodatevarious sensitivity curves as may be required in various sensorapplications. In general this is accomplished by means of a detectorcircuit for use in combination with a semiconductor sensor of thevariable resistance type wherein feedback means are provided between theoutput and the input of a control circuit thereby to effectivelyincrease the sensitivity of the overall detector system to toxic gaslevel once an alarm condition has been reached. In accordance with thepreferred embodiment of the invention hereinafter described in detailthe control circuit includes an operational amplifier functioning as acomparator to control the on and off states of a transistor switch, anda feedback circuit comprising a resistor, which may be variable, betweenthe primary circuit of the transistor switch and one of the inputs ofthe operational amplifier thereby to facilitate the variable hysteresischaracteristic in a simple and highly economical fashion.

In accordance with a third feature of the invention an input circuit ofnovel design is provided to eliminate the inherent droop in line voltagetypical of the prior art devices when the alarm condition obtains and,in fact, to provide a slight increasein the energizing signal to thesensor thus to effectively decrease sensor response and purge the sensorarea by increasing the temperature and oxidation rate of the sensordevice. In the preferred embodiment to the invention hereinafterdescribed in detail, this specific objective is met and accomplished bymeans of an input circuit in the form of an autotransformer havingtapped primary winding and a pair of secondary windings, one of thesecondary windings being employed for the purpose of energizing thesensor and the other secondary winding being employed to generate areference voltage. The tapped primary winding is adapted to be connectedto a supply line and is also connected to the alarm device, such as abuzzer or light or combination of such devices, through a relaycontrolled switch. When the alarm condition obtains and the relaycontrolled switch closes, the effect of the autotransformer is toincrease current through the secondary winding which supplies theenergizing signal to the sensor thus to increase the temperature of thesensor and effectively decrease its response to the toxic gas level.

These and other features and advantages of the invention will be bestunderstood by reference to the following specification which describes aspecific embodiment of the invention in detail.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramof an illustrative embodiment of the invention and employing a metaloxide semiconductor sensor device;

FIG. 2 is a representation of the hysteresis effect which is provided bythe circuitry of FIG. 1; and

FIG. 3 is a block diagram of an air supply system for a plurality ofusers wherein the sensor and associated circuitry of FIG. 1 are employedto indicate and constantly monitor the quality of air being delivered toa plurality of recipients.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT Looking now to FIG. 1the specific and illustrative embodiment of the invention is shown tocomprise a circuit including a metal oxide semiconductor sensor whichpresents a variable resistance characteristic when exposed to toxicgases such as carbon monoxide, propane, butane, alcohol, gasoline andother hydrocarbons. The sensor 10 is connected to an input or energizingcircuit 12 hereinafter described in detail to provide a suitable ACenergization of the sensor as well as to provide a DC signal forenergization of a bridge circuit 13. The sensor 10 is further connectedto an output or control circuit 14 which responds to the variableresistance characteristics of the sensor to control the condition of analarm device 16. As previously described the alarm device 16 may be anyof a large number of devices including lights, buzzers, bells, stripchart recorders and other indicating and data collecting devices.

The sensor 10 is preferrably a metal oxide semiconductor device having aresistance characteristic which changes in the presence of acontaminating gas such as those mentioned above. In addition suchsemiconductor devices are highly responsive to the magnitude of theenergizing signal; ie, an increase in the magnitude of the energizingsignal quantity results in an increase in the temperature of the sensorand this temperature increase results in a higher rate of oxidation ofthe toxic gas being sensed thus to appear as a decrease in sensitivityor response. A suitable device is available from Figaro Engineering ofOsaka, Japan and is described in the US. Pat. to Taguchi, No. 3,631,436.The schematic circuit representation of FIG. 1 is taken from thedisclosure of the Taguchi patent. The metal oxide semiconductor deviceavailable from Figaro Engineering exhibits a normal resistance on theorder of 100,000 ohms in air at room temperature and a resistance ofapproximately 100 ohms in a heavy alcohol vapor. Accordingly thesensitivity and response range of such devices is particularly suitablefor use in the present invention.

Describing the circuit of FIG. 1 in greater detail, the input circuit 12is shown to comprise an autotransformer having a tapped primary winding18 adapted to be connected across a l 10 volt AC supply by means ofterminals 20 and 22. The autotransformer further comprises separatesecondary windings 24 and 26 of which the secondary winding 24 isconnected through a current limiting resistor 28 to the input side ofthe semiconductor sensor 10. The other side of the secondary winding 24is connected to ground 30 as shown. The output side of the sensor isrepresented by line 32 and carries the electrical signal quantity of theillustrative embodiment which represents the toxic gas level beingsensed. This signal is applied to the bridge circuit 13 which may beconsidered as part of output or control circuit 14 as hereinafterdescribed.

Secondary winding 26 is connected through a conventional rectifyingbridge made-up of diodes 34 to an output line 36 which is applied to thebridge circuit 13. A capacitor 38 is connected between line 36 andground 30 to filter out any AC component which might be present and toprovide a relatively uniform constant level DC signal as the bridgeenergization signal quantit The bridge circuit 13 comprises one legeffectively made-up of the variable resistor 42 and the sensor 10, thesupply for the leg of the bridge including resistor 42 coming from thediode rectifier bridge by way of line 36. The other leg of the bridgecircuit comprises fixed and equal value resistors 44 and 46, themidpoint or junction 48 of which is representative of the point at whichthe reference signal is obtained.

Output or control circuit 14 comprises as the primary active elementthereof an operational amplifier 40 which functions as a voltagecomparator and which is provided with a positive input connected tojunction point 48 between the resistors 44 and 46 in the reference legof the bridge circuit previously described. The negative input ofoperational amplifier 40 is connected to the variable leg of the bridgecircuit thus to receive the output of the sensor 10. The values of theresistors 42, 44, and 46 are set such that the inputs to the operationalamplifier 40 are normally unbalanced; ie, the negative input voltage isgreater than the positive input and, thus, a low voltage signal appearson output 50 of the amplifier.

The signal on output 50 is applied directly to the base or controlelectrode of the transistor switch 52 shown in FIG. 1 to be of the pnptype. The emitter-collector circuit of transistor switch 52 is connectedin series with a supply source Vbb, the parallel combination of a relaycoil RS1 and diode 54, and a series resistor 56 which is connected toground 30. The low voltage output of the operational amplifier 40appearing on line 50 is effective to maintain the transistor switch 52in the conductive condition as long as no toxic gas contamination isdetected by the sensor 10.

As indicated by the reference characters RS1 and $1 on the relay coiland switch, respectively, the flow of current from the source Vbbthrough the relay coil RS1 and the transistor 52 operates to maintainthe switch S1 between the center tap 58 of the transformer primarywinding 18 and the alarm device 16 in the open condition. Any failure ofcurrent to flow through the relay coil RS1 results in the immediateclosure of switch S1. When the switch S1 is open, no current flowsthrough the alarm device 16 and, accordingly, no warning or indicationof toxic gas presence is given. However, any failure of current to flowthrough the coil RS1 results in the immediate closure of switch S1 thuscausing the current to flow through the alarm device 16 to indicate theexposure of the sensor 10 to a toxic gas. It can be seen that theautomatic closure of switch S1 upon failure of current to fiow throughthe relay coil RS1 is a fail-safe feature in that any loss of signal orcircuit failure in the system of FIG. 1 between the secondary windingsof the transformer and the output circuit of transistor 52 will resultin the indication of an alarm condition.

As previously mentioned, the bridge circuit comprising resistors 42, 44,and 46 and the sensor 10 is initially unbalanced; ie, the resistor 42 isset so that the voltage on line 32 is greater than the voltage appearingat point 48. Accordingly, the operational amplifier 40 produces a lowoutput which is at, or near, ground level potential. A negativebase-emitter bias on transistor 52 renders it conductive and causescurrent to flow through the relay coil RS1. This maintains switch 81open and holds off the alarm indication. Should any variation in linevoltage occur, such variation is reflected equally into the secondarywindings 24 and 26 and, accordingly, the bridge remains unbalanced sincethe sensor signal and the reference signal quantities remain in the sameratio. On the other hand, should sensor 10 be exposed to a toxic gas ofsufficient concentration, the

voltage on line 32 is caused to decrease until it equals the voltage atpoint 48. Atthis time the operational amplifier 40 switches to a high"output condition rendering transistor 52 nonconductive. This terminatesthe flow of current through the relay coil RS1 and permits switch S1 toclose. With switch S1 closed, current flows from terminal 20 to primarywinding 18, center tap 58, switch S1, and alarm 16 back to terminal 22,thus, turning on the alarm.

The semiconductor sensor is completely selfrecovering and when the toxicgas condition has been cleared theself-purging action hereinafterdescribed in greater detail terminates the alarmcondition and restoresthecircuit of FIG. 1 to the ready condition. It may, of course, beadvisable in certain instances to include a recording device in alarml6to indicate the occurrence of the alarm condition over some period ofmonitoring. Alternatively it may be desirable to establish a holdingfeature in the alarm 16 so that it must be reset by some prespecifiedact.

Looking again to FIG. 1 the operational amplifier 40 is provided with apositive feedback path 60 including a resistor Rb which is connectedbetween the emitter of transistor 52 and the positive input of theamplifier. Accordingly the failure of current to flow through theemitter-collector circuit of transistor 52 results in a voltage increaseat the positive input of the amplifier thus giving rise to a hysteresiscondition which is illustrated in the curve of FIG. 2.

Looking to FIG. 2 it can be seen that the curve 62 has a broadhysteresis loop configuration indicating that the alarm condition isgiven at a high toxic gas concentration (in this case CO) whereas thealarm condition remains in effect until the toxic gas concentration hasbeen reduced to a level which is substantially below the initialtriggering level. By varying resistor Rb in the positive feedback path50 the width of the loop in curve 62 may be varied between zerohysteresis and infinite hysteresis depending on the desires andobjectives of the individual circuit designer and user.

Still referring to FIG. 1 another feature of the invention is the use ofthe autotransformer in the input circuit 12 having split secondarywindings 24 and 26 and the center tapped primary winding 18. Althoughthe scientific explanation for the phenomenon to be describedimmediately hereinafter is not fully understood the benefits of thephenomenon are quite clear. It has been found that the closure of switchS1 to establish the alarm condition has the effect of couplingadditional flux through the core of secondary winding 24 thus producingincreased current flow through the semiconductor sensor 10. This in turnhas the effect of heating up the sensor 10 and decreasing its effectivesensitivity to toxic gas concentrations. As will be apparent to those ofordinary skill in the art, the opposite effect which is normallyencountered when line voltage droops from alarm turn-on can produce acondition of instability where the alarm tends to be locked on" untilthe circuit is broken. In addition the added heating affect of thesensor 10 tends to produce a purging action; ie, the residual gasconcentration around the sensor is oxidized at an increased rate.Clearly there are other circuit configurations which might give rise tothis increased current flow condition and accordingly while the circuitconfiguration of FIG. 1 is preferred and represents the best mode ofcarrying out the invention now known to the inventor it is to beunderstood that the invention is not limited to the exact structureillustrated in FIG. 1.

Looking now to FIG. 3 there is shown a system 64 for supplying air to aplurality of users of life support sys tems; ie, persons who are workingin a contaminated air atmosphere and who must wear breathing apparatus.In FIG. 3 the air supply 66 may take the form of a compressor or airpump connected through a manifold type particulate filter 68 having aplurality of output lines or hoses 70, 71, 72 and 74. A suitableparticulate filter is available from the E. D.Bullard Company ofSausalito, California.

As shown in FIG. 3 a semiconductor toxic gas sensor 76 of the typedesignated by the reference character 10 in FIG. 1 is suitably disposedin the airline 74 so as to constantly monitor the quality of air beingsupplied through the life support system to the various users thereof.It is necessary to monitor only one of the several life support lines70, 71, 72 and 74 inasmuch as all receive air from a common supply.Sensor 76 is connected to an electronic unit 78 which in turn isconnected to an alarm device 80 to indicate the presence of a toxic gasof predetermined concentration in the air supply of the life supportsystem. The electronic unit 78 may, of course, take the form illustratedin FIG. 1 and accordingly the alarm device 80 may correspond in allessential functional respects to the alarm device 16 of FIG. 1.

Various other uses of the sensor system of FIG. 1 and equivalencethereof will occur to those skilled in the art.

It is to be understood that the description given above is illustrativein character and is not to be construed as limiting the subjectinvention to the exact construction and circuit arrangements shown.

Having thus described my invention, I claim:

1. Apparatus for indicating the presence of a toxic gas comprising, asolid state sensor having a relatively large nominal resistance butbeing responsive to the presence of said toxic gas to present asubstantially reduced electrical resistance characteristic, bridgecircuit comprising as a first leg the series combination of said sensorand a variable resistor and as a second leg the series combination oftwo reference resistors, the first and second legs being connected inparallel, input circuit means including first means connected directlyto said sensor for energizing said sensor with electrical energy of apredetermined magnitude for producing an electrical signal quantityrelated to the resistance characteristic of said sensor, said inputcircuit means further including second means connected across both ofthe first and second legs for energizing the bridge circuit and forproducing a reference quantity, control circuit means includingcomparator means having separate inputs connected to the mid-points ofsaid first and second legs for comparing said electrical signal quantityto said reference quantity and for producing an output signal quantitywhen said electrical signal quantity bears a predetermined relationshipto said reference quantity, and indicator means for indicating an alarmcondition connected to be controlled by said output signal quantity forindicating the presence of said gas, said input circuit means includinga transformer having a tapped primary winding and first and secondsecondary windings, said tapped primary winding being connected to saidindicator means for energizing said indicator means under the control ofsaid output signal quantity, said first secondary winding beingconnected to energize said sensor, and said second secondary windingbeing connected by circuit means to said reference resistor to producesaid reference quantity, whereby the relative energization of said firstand second legs is constant irrespective of variations in the voltageapplied to said primary winding.

2. Apparatus as defined in claim 1 wherein said control circuit meansfurther includes switch means having conductive and nonconductive statesand being connected to receive said output signal quantity to becontrolled thereby and relay means connected in circuit with said switchmeans and a source of electrical energy, applied to said sensor saidrelay means being peratively connected with said indicator means forenergizing said indicator means whenever said switch means is in one ofsaid conditions.

3. Apparatus as defined in claim 2 wherein said relay means energizessaid indicator means whenever said switch means is in the nonconductivecondition.

4. Apparatus as defined in claim 1 wherein said sensor is a solid statesemiconductor device having a response characteristic to the presence ofsaid toxic gas which is related to the magnitude of the energizingelectrical energy applied thereto, said input circuit means includingmeans for increasing the magnitude of said energizing energy applied tosaid sensor whenever said indicator means indicates an alarm condition.

5. The apparatus as defined in claim 1 further including feedback meansconnected between the input to said comparator means connected to saidsecond leg and the output of said control circuit means forautomatically varying said predetermined level upon activation of saidindicator means when the presence of said gas is detected by said sensorwhereby the toxic gas level for terminating the alarm condition is lessthan said predetermined level.

6. Apparatus for indicating the presence of a toxic gas comprising, asolid state sensor having a nominal relatively large resistance butbeing responsive to the presence of said toxic gas to present asubstantially reduced electrical resistance characteristic, inputcircuit means connected to said sensor for energizing said sensor withelectrical energy of a predetermined magnitude, control circuit meanshaving an input and output, said input being connected to said sensorfor receiving an electrical signal quantity from said sensor related tothe resistance characteristic thereof, said control circuit meansproducing on said output an output signal quantity when said electricalsignal quantity on said input reaches a predetermined level, and alarmmeans for indicating an alarm condition connected to be controlled bysaid output signal quantity for indicating the presence of said gas,said input circuit means further including means for increasing themagnitude of said energizing energy applied to said sensor whenever saidalarm means indicates an alarm condition.

7. Apparatus as defined in claim 6 wherein said input circuit meanscomprises a transformer having a primary winding and at least a firstsecondary winding, said first secondary winding being connected toenergize said sensor, and means connected between said primary windingand said alarm means for energizing said alarm means under the controlof said output signal.

1. Apparatus for indicating the presence of a toxic gas comprising, asolid state sensor having a relatively large nominal resistance butbeing responsive to the presence of said toxic gas to present asubstantially reduced electrical resistance characteristic, bridgecircuit comprising as a first leg the series combination of said sensorand a variable resistor and as a second leg the series combination oftwo reference resistors, the first and second legs being connected inparallel, input circuit means including first means connected directlyto said sensor for energizing said sensor with electrical energy of apredetermined magnitude for producing an electrical signal quantityrelated to the resistance characteristic of said sensor, said inputcircuit means further including second means connected across both ofthe first and second legs for energizing the bridge circuit and forproducing a reference quantity, control circuit means includingcomparator means having separate inputs connected to the mid-points ofsaid first and second legs for comparing said electrical signal quantityto said reference quantity and for producing an output signal quantitywhen said electrical signal quantity bears a predetermined relationshipto said reference quantity, and indicator means for indicating an alarmcondition connected to be controlled by said output signal quantity forindicating the presence of said gas, said input circuit means includinga transformer having a tapped primary winding and first and secondsecondary windings, said tapped primary winding being connected to saidindicator means for energizing said indicator means under the control ofsaid output signal quantity, said first secondary winding beingconnected to energize said sensor, and said second secondary windingbeing connected by circuit means to said reference resistor to producesaid reference quantity, whereby the relative energization of said firstand second legs is constant irrespective of variations in the voltageapplied to said primary winding.
 1. Apparatus for indicating thepresence of a toxic gas comprising, a solid state sensor having arelatively large nominal resistance but being responsive to the presenceof said toxic gas to present a substantially reduced electricalresistance characteristic, bridge circuit comprising as a first leg theseries combination of said sensor and a variable resistor and as asecond leg the series combination of two reference resistors, the firstand second legs being connected in parallel, input circuit meansincluding first means connected directly to said sensor for energizingsaid sensor with electrical energy of a predetermined magnitude forproducing an electrical signal quantity related to the resistancecharacteristic of said sensor, said input circuit means furtherincluding second means connected across both of the first and secondlegs for energizing the bridge circuit and for producing a referencequantity, control circuit means including comparator means havingseparate inputs connected to the mid-points of said first and secondlegs for comparing said electrical signal quantity to said referencequantity and for producing an output signal quantity when saidelectrical signal quantity bears a predetermined relationship to saidreference quantity, and indicator means for indicating an alarmcondition connected to be controlled by said output signal quantity forindicating the presence of said gas, said input circuit means includinga transformer having a tapped primary winding and first and secondsecondary windings, said tapped primary winding being connected to saidindicator means for energizing said indicator means under the control ofsaid output signal quantity, said first secondary winding beingconnected to energize said sensor, and said second secondary windingbeing connected by circuit means to said reference resistor to producesaid reference quantity, whereby the relative energization of said firstand second legs is constant irrespective of variations in the voltageapplied to said primary winding.
 2. Apparatus as defined in claim 1wherein said control circuit means further includes switch means havingconductive and nonconductive states and being connected to receive saidoutput signal quantity to be controlled thereby and relay meansconnected in circuit with said switch means and a source of electricalenergy, applied to said sensor said relay means being operativelyconnected with said indicator means for energizing said indicator meanswhenever said switch means is in one of said conditions.
 3. Apparatus asdefined in claim 2 wherein said relay means energizes said indicatormeans whenever said switch means is in the nonconductive condition. 4.Apparatus as defined in claim 1 wherein said sensor is a solid statesemiconductor device having a response characteristic to the presence ofsaid toxic gas which is related to the magnitude of the energizingelectrical energy applied thereto, said input circuit means includingmeans for increasing the magnitude of said energizing energy applied tosaid sensor whenever said indicator means indicates an alarm condition.5. The apparatus as defined in claim 1 further including feedback meansconnected between the input to said comparator means connected to saidsecond leg and the output of said control circuit means forautomatically varying said predetermined level upon activation of saidindicator means when the presence of said gas is detected by said sensorwhereby the toxic gas level for terminating the alarm condition is lessthan said predetermined level.
 6. Apparatus for indicating the presenceof a toxic gas comprising, a solid state sensor having a nominalrelatively large resistance but being responsive to the presence of saidtoxic gas to present a substantially reduced electrical resistancecharacteristic, input circuit means connected to said sensor forenergizing said sensor with electrical energy of a predeterminedmagnitude, control circuit means having an input and output, said inputbeing connected to said sensor for receiving an electrical signalquantity from said sensor related to the resistance characteristicthereof, said control circuit means producing on said output an outputsignal quantity when said electrical signal quantity on said inputreaches a predetermined level, and alarm means for indicating an alarmcondition connected to be controlled by said output signal quantity forindicating the presence of said gas, said input circuit means furtherincluding means for increasing the magnitude of said energizing energyapplied to said sensor whenever said alarm means indicates an alarmcondition.